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California tgional Water Qualityjontrol Board Central Valley Region ;`` Peter M.Rooney Sacramento Main Office Ed J.Schnabel Secrnlary for - Internet Address: http://www.swrcb.co.gov/—rwqcb5/home.htmi " " Chair Fnvironmental 3443 Routier Road,Suite A,Sacramento,California 95827-3003 Protection Phone(916)255-3000•FAX(916)255-3015 TO: Gordon L. Boggs FROM: Mark R. List UST Program Supervisor Engineering Geologist - I DATE: 19 August 1998 SIGNATURE: SUBJECT: FILE REVIEW FOR CLOSURE CONCURRENCE, FORMER UNOCAL STATION #5421, 7303 PACIFICA VE.,STOCKTON, SAN JOAQUIN COUNTY At the request of the San Joaquin County Environmental Health Department (SJCEHD), a file review was completed to determine if Board staff agree with the SJCEHD determination of no further action required at the above referenced site. The following information was obtained during my review of the i Regional Board casefile and the SJCEHD casefile, and following several discussions with Mr. Harlin Knoll, the SJCEHD caseworker for the 7303 Pacific Ave. site. Background: Two generations of USTs have been in place at this site. The first set included two 10,000-gallon gasoline USTs, and one 280 waste oil tank. All three of the first generation tanks were removed on 17 November 1989. The second generation tanks, two 12,000-gallon, double walled, gasoline USTs, and one 520-gallon waste oil UST, were installed in early 1990, and were removed on 15 April 1998. Soil contamination was associated with both generations of tanks. Additionally, a septic system and disposal field was located on-site and clean-up of this contamination is the responsibility of Fed EPA. The first generation tank pit was overexcavated to approximately 25.5 feet below ground surface (bgs), in the southern portion of the pit. Confirmation soil samples indicate that a significant portion of the contaminated soil was removed during overexcavation activities. Some residual TPHg and benzene - ' contariunated'soil re>nains-in place,as evidenced by excavation-sidewall�soil sample.analytical results,-- -_ collected at 17 feet bgs. The results for sample P2(17) show 7,200 ppm TPHg, and 170 ppm benzene . The extent of this contamination may be confined to the immediate vicinity of the sldewall. Monitoring well MW-3 was installed very near the location of sample P2(17). Soil samples collected at 30.5 feet bgs evidenced 0.088 ppm benzene, however, benzene;was not detected above the laboratory reporting limit, in this boring,below the 30.5 foot depth,to the'total depth of 59 feet. TPHg was not detected above the laboratory reporting limit, for any sample in the boring for MW-3- The boring for j MW-3 was not sampled above 30.5 feet bgs due to the presence of tank backfill material (pea gravel)to a depth of approximately 27 feet bgs. An unknown mass of contamination is present at approximately 17 feet bgs, in the vicinity of MW-3. The vertical extent of this contamination appears to be defined by the MW-3 soil boring. 9. i California Environmental Protection Agency Z,D Recycled Paper